1. Field of the invention
The present invention relates to a data communication system between two different data transmission systems for the two different groups of CPUs.
2. Description of the Prior Art
A highly sophisticated apparatus, such as an electrophotographic copying machine, has a number of mechanical arrangements, such as a paper supplier, an enlargement setting device, a paper size selecting device, paper sorter, etc. These arrangements are operated in a predetermined timed reltionship in a quite complex manner. To accomplish the sophisticated serial operations of the apparatus in a controlled manner, arrangements have their own CPUs (central processing unit) called slave CPUs, which are governed by a central CPU called a host CPU. The data transmission between the host CPU and the number of slave CPUs is carried out, e.g., by a system which is disclosed in a U.S. patent application Ser. No. 569,953 filed Jan. 11, 1984 assigned to the same assignee as the present application. This system is herein referred to as a sequence data transmission system in which the slave CPUs take turns in a predetermined programmed sequence to transmit data from each slave CPU to the host CPU through a common data bus extending in a tree structure from the host CPU to each of the slave CPUs.
Another sophisticated apparatus, such as a word processor, has a group of CPUs which are provided for controlling the components, such as key board, CRT, memory unit and printer. In this type of apparatus, all the CPUs operate individually, i.e., without being governed by a certain CPU. Because the amount of data to be processed in the CPUs in the word processor is much greater than that for the copying machine, the word processor employs a high speed data transmission system, e.g., a data transmission system disclosed in U.S. Pat. No. 4,603,383 also assigned to the same assignee as the present application. This system is herein referred to as a registration data transmission system in which the CPUs take turns to have an access to an externally provided RAM for registering data about an operation of the apparatus. The registered data will be transferred to another CPU when it is that other CPU's turn to have an access to the RAM. The operation data to be transferred may include a request for a direct data transmission from one CPU to another CPU through a data bus connected directly between the CPUs. Such a direct data transmission is necessary for sending a large amount of data, such as the word data for the word processing.
A problem arises when these two apparatuses having different data transmission systems are to be assembled together with the need of a mutual data exchange. Such an assembly is, for example, when the word processor employing a laser printer is to be assembled together with a copying machine. In this case, there will be a desire to use, for example, the paper supplier and/or the paper sorter provided in the copying machine for efficiently making prints by the laser printer for the word processor. In this case, the problem is how to communicate operation data from one data transmission system to another data transmission system.